Windows: If you miss the transparent Aero Glass interface of ye olde Windows 7, free tool WinAeroGlass can bring it back in two clicks.

Right now WinAeroGlass is in alpha stage, and has no UI—you just double-click on its icon and it'll make all your windows' title bars transparent. It also doesn't blur any of the transparency like Windows 7 used to do, but the developer is working on this right now, so hopefully we'll see this feature available in a new version soon. To turn off the glass, just log out and log back in—simple as that.